This invention relates to cold-crucible induction melting techniques and apparatuses therefor. More particularly, this invention is directed to the use of a horizontal floating-zone cold crucible for at least levitating and locally melting a material by induction heating to refine and/or analyze the material, and particularly for the purpose of removing insoluble inclusions while leaving desired alloying constituents in tact.
Vertical cold-crucible induction levitation melting techniques are known, an example of which is U.S. Pat. No. 3,702,368 to Hukin. With such techniques, a material is placed in a vertically-oriented metal crucible, melted and then slightly levitated out of contact with the crucible by a radio frequency field generated by an induction coil surrounding the crucible. The walls of the crucible are water-cooled and segmented, the latter of which enables induction heating to occur through the metal crucible walls by interrupting induced current flow in the walls that would otherwise attenuate the induction coil field.
Melting by induction levitation has also been used to purify materials such as silicon (e.g., UK Patent Application 2207061) and high-temperature reactive metals such as titanium, niobium and chromium and their alloys by a process known as floating-zone refining. The chemical reactivity of titanium, niobium and chromium can degrade conventional oxide crucibles as a result of a high negative free energy of the reactive metal oxide formation. While melting and casting operations can be performed in an inert atmosphere or vacuum to avoid reactions with gaseous oxygen and nitrogen, oxygen and nitrogen are generally nonetheless available either in the form of impurities in the processing environment or from the crucible. Significant degradation of the crucible and contamination of a reactive melt becomes more likely as the melting temperature of the alloy and the concentration of reactive elements in the alloy increase. For these reasons, floating-zone refining has been used to purify reactive materials, during which a molten zone is traversed from one end of the material to the other, where elemental impurities become concentrated as a result of solubility differences of the impurities between the liquid and solid phases of the material.
Zone refining can be categorized as either horizontal floating-zone or vertical floating-zone, depending on the orientation of the material and the direction in which the molten zone moves during refining. In both techniques, the material is completely surrounded by the crucible walls in order to contain and achieve a substantially uniform temperature in the molten zone. While horizontal and vertical floating-zone refining techniques have been developed to produce very pure ingots of high-temperature elemental materials that contain soluble elemental impurities, they have not been successfully employed for removing minimal solubility inclusions from alloys. Inclusions of interest include oxides, carbides, nitrides, silicides, carbonitrides, oxynitrides and oxycarbides, which occur in alloys such as nickel-based and cobalt-based superalloys and titanium alloys. Accordingly, it would be desirable if a technique were available for enabling zone refining of high-temperature alloys, including alloys containing reactive elements, for the purpose of removing insoluble inclusions from an alloy but not its desired alloying constituents. It would also be desirable if such a technique employed an improved crucible design that enabled additional processes to be carried out on the alloy being refined.